Contrast-dependent and contrast-independent spatial summation of primary visual cortical neurons of the cat.

Recent studies have reported that the size of the classical receptive field (CRF) and the extent of spatial summation of V1 neurons depend on stimulus contrast. We reexamined these properties for 48 V1 neurons in the cat and found that all the cells had a constant CRF size, whereas their spatial summation properties can be contrast dependent (CD) or contrast independent (CID). Of the 29 CD cells, 17 showed facilitatory summation at low contrast (10%), but suppressive summation at high contrast (80%); the other 12 showed weak suppressive summation at low contrast, whereas the strength of suppression increased significantly at high contrast. The 19 CID cells showed similar facilitative (CIDf) or suppressive (CIDs) summation at low and high contrast, without changes in shape and/or peak location. We successfully labeled 11 CD cells and 10 CID cells with biocytin. The morphological results demonstrated that all the labeled CD cells were pyramidal cells, whereas all labeled CID cells were nonpyramidal cells, in which the CIDf cells were spiny stellates and the CIDs cells, smooth interneurons. There is thus a global distinction between summation properties of pyramidal and nonpyramidal cells, and between the smooth and spiny nonpyramidal cells as well.